1. Field of the Invention
The present invention relates to a video signal decoder and more particularly to a video signal detecting circuit for detecting synchronous signals included in a video signal.
2. Description of the Background Art
Generally, a circuit for detecting a TV (television) or similar video signal is sometimes supplied with a video signal having a format other than the NTSC (National Television System Committee) or similar standard format or sometimes with no video signals. Such a video signal detecting circuit must generate vertical synchronous signals at adequate timing without regard to the format of the input video signal or even when no video signals are input.
Japanese patent laid-open publication No. 341304/1999 discloses a video signal detecting circuit that is a partial solution to the above problem. The video detecting circuit taught in this document distinguishes three different modes of an input video signal from each other and generates vertical synchronous signals in a particular manner for each mode. More specifically, in a first mode in which the circuit receives a standard format of video signal, it decodes vertical synchronous signals with a line counter. In a second mode in which the circuit receives a video signal other than the standard format of video signal, it detects vertical synchronous signals without using the line counter while outputting the signals. Further, in a third mode in which the circuit receives no video signals, it outputs vertical synchronous signals in a free-running fashion to cause a blank screen to be displayed.
The video signal detecting circuit taught in the above-mentioned laid-open publication has some problems left unsolved, as will be described hereinafter. Generally, so long as an input video signal has a standard or a nearly standard format, the number of horizontal scanning lines changes little. Also, with a video signal having a format other than the standard format, the horizontal scanning lines may accurately be counted. In such a condition, the video detecting circuit disclosed by the aforementioned Japanese publication automatically selects the first mode or standard mode in order to accurately decode the video signal. Pictures represented by the decoded video signal will appear natural to eye when displayed on a video monitor.
However, pictures are sometimes lowered in quality when displayed on a video monitor. This is because even when the number of horizontal scanning lines meets the standard, the number of pixels included in each of the horizontal scanning lines is sometimes not constant. As for the NTSC standard, for example, ITU601 prescribes that the number of pixels for a single scanning line should be 858. In this case, if one line has 857 pixels while another line has 859 pixels by way of example, then the vertical edge of a picture area is viewed jagged or the vertical lines shown in a picture are viewed zigzag, resulting in low picture quality.
As for the first mode, the video signal detecting circuit disclosed in the Japanese publication counts horizontal scanning lines with the line counter to thereby decode vertical synchronous signals, as stated earlier. However, under the above-stated situation in which the number of pixels for a single horizontal scanning line does not meet the standard although the number of scanning lines for a single picture meets it, it is likely that the vertical synchronous signals derived from the count of the line counter are not coincident in timing with the vertical synchronous signals included in the input video signal. This prevents the video signal from being accurately decoded.
Further, with the video signal detecting circuit disclosed by the Japanese publication, a free-running function is available with no signals received, and assigned to the third mode or start mode. The free-running function, however, cannot distinguish a condition where a video signal is not input due to the disconnection of a cable from a condition where a video signal is input but cannot be decoded due to excessive noise.